This invention relates to a polling communication method for exchanging predetermined data by specifying a terminal receiving requests in a short time period. The terminal centers are connected by a common transmission line (a communication line). Each asks the other whether or not there are any requests at the terminal, especially when a collision of requests occurs at the terminals between the centers and the terminals.
There are, in general, various communication systems, such as a communication system for information in a community, a communication system for education, e.g., a school education communication system, a a guide service, like a wide service CATV system.
FIG. 4 schematically shows an example of a prior art videotex system, mainly for a guide service. This communication system is composed by connecting a center A for storing predetermined information to a number of terminals C.sub.1 to C.sub.n through signal transmission lines B.
In the communication system of FIG. 4, when the terminal C.sub.1 requests of center A, a still picture NO. z, the center A identifies the terminal C.sub.1 and the still picture number NO.z, and then transmits still picture number NO.z to the terminal C.sub.1.
Therefore, in the communication system of FIG. 4, when any of the terminals C.sub.1 to C.sub.n requests a predetermined service of center A through the signal transmission line B, and the center A intends to execute a service responsive to the requests, the center A must identify which of the terminals requests which kind of content of the service.
In the polling of the communication system, the center A asks what kind of service is requested at the respective terminals C.sub.1 to C.sub.n, and specifies the requesting terminal and the content of the request. Thus, the center A can process the above-mentioned request.
In this polling method, the most simple one is a sequential polling method.
According to the sequential polling method, the center A sequentially asks the terminals C.sub.1 to C.sub.n the contents of their requests.
There is an other prior art polling method, a binary looking-up without forward address, as shown in FIG. 5.
In the polling method in FIG. 5, the center first asks whether or not there is a request to terminals C.sub.1 to C.sub.n of an entire range L.sub.1. When the center identifies a request, the center asks whether or not there is a request at the terminals of a range L.sub.2 =1/2.times.L.sub.1, and when there is no request at the terminals of the range L.sub.2, the center judges that there is a requesting terminal in the remaining range L.sub.2 '.
Thus, the range of requesting terminals is sequentially narrowed to a half, like L.sub.3, L.sub.4, . . , L.sub.n, and the center continues polling until the requested terminal is specified.
In FIG. 5, the mark "o" indicates "there is a request", and the mark "x" indicates "there is no request".
The number n of pollings in the polling method in FIG. 5 is represented as below. EQU n=1+log.sub.2 N
where N: the number of terminals.
When N is large, such as, for example, N=100,000 the terminal requesting can be specified by (n-18) times.
Therefore, in the polling method in FIG. 5, the requestial procedure is complicated, but the number of necessary pollings can be reduced to shorten the total request processing time.
There is a polling method according to a binary looking-up with forward address that accelerates polling.
In this polling method, the terminal not only replies merely whether or not there is a request but also answers if there is a request with a self-address to the center. Thus, the center can specify the terminal requesting at the moment that the requesting terminal makes a request.
For example, assume that there are requests at terminals of addresses No. 1 and No. 50,000, the following procedure is executed.
At first polling, the center asks whether or not there is a request to all the terminals.
In this case, the terminals of No. 1 and No. 50,000 reply that they have requests, but since these two requests collide, even if the center can identify that there are requests, the center cannot identify which of the terminals have made them.
In case of the second polling, the center asks whether there are requests to the terminals of the range of No. 1 to No. 50,000.
At this time, the center can identify only that there are requests.
In case of the third polling, the center asks whether or not there are requests to the terminals of the range of No. 1 to No. 25,000.
At this time, since there is only one terminal of No. 1 requesting, a collision of the requests (replying data) does not occur and the center can identify the terminal of No. 1 requesting.
As described above, when the center thus identifies the terminal of address No. 1 requesting, the center transmits predetermined information to the requesting terminal.
However, the above-mentioned polling methods have the following drawbacks.
For example, since the most simple sequential polling method in the prior art has the number of pollings corresponding to the number of the terminals, it takes a long time to specify the requesting terminal when there are a number of terminals.
Since the polling method of FIG. 5, while desirable as compared with the sequential polling method, has no forward address, it takes a long time to specify the requesting terminal.
The most desirable binary, looking-up, polling method with forward address in the prior art can specify the requesting terminal at a high speed when the rate of requests is low, but when the rate of requesting terminals is increased so that requests from the terminals frequently collide, the polling time per request is lengthened, and the total polling time is increased that much.